1. Field of the Invention
The present invention relates to a projection exposure apparatus and a method for manufacturing devices such as a semiconductor device including an IC or an LSI, an image pickup device including a CCD, a dispay device including a liquid crystal panel, a magnetic head, etc. The projection exposure apparatus and the device manufacturing method are advantageous for a case where any of these devices is to be manufactured by lithography to have a high degree of integration by projecting an electronic circuit pattern formed on the surface of a mask or a reticle (hereinafter referred to as a reticle) on a wafer through a projection optical system to carry out an exposure or a scanning-exposure action on the wafer. Particularly, the projection exposure apparatus and the device manufacturing method are advantageous for a case where a silicon wafer is to be exposed with an electronic circuit pattern of the reticle by precisely aligning and adjusting the position of the reticle and that of the wafer to each other to obtain a high degree of integration.
2. Description of Related Art
In manufacturing a semiconductor device, a liquid crystal element, etc., by photolithography, a projection exposure apparatus which is called a stepper is used. The projection exposure apparatus is arranged to carry out an exposure by projecting through a projection optical system a pattern formed on a reticle onto a photosensitive substrate which is a wafer or a glass plate coated with a photoresist or the like.
The art of manufacturing semiconductor devices to have a minute and fine circuit pattern has saliently advanced of late. Efforts are being made to have a circuit pattern more minutely defined, for example, down to such a line width measuring less than 0.25 .mu.m. In respect of those efforts, an art of making optical exposures typically embodied in the stepper is playing the most important role. The performance of a projection lens, i.e., a projection optical system, which is usable as an index for the performance of the art of optical exposure, is considered roughly in three aspects, including a shorter wavelength, a larger image plane and a larger NA (numerical aperture). With respect to having a shorter wavelength, efforts are now ardently exerted to develop as a technique of the next generation a lithographic art of using a laser beam which is obtainable from an ArF excimer laser.
The optical exposure method for manufacturing semiconductor elements (devices) are required to satisfy various factors (requirements) which include, besides a high resolution, a high degree of precision of positional alignment, i.e., position adjustment to each other, of patterns to be superposed on each other in many layers.
Of known position adjusting methods for this purpose, a method called "global alignment" is popularly in use. The error to be made by the global method can be roughly divided into two error components. One is an intershot error component which is an error between one printing shot (exposure shot) and another and an intrashot error component which is an error made within each shot of exposure. Since the size of an image plane is tending to become larger of late, how to suppress the intrashot error component is a serious problem these days.
For example, if there is an error of 2 ppm with respect to magnification or perpendicularity for an image plane size of 22 mm, the alignment error becomes as follows: EQU 22 mm.times.2 ppm=44 nm.
The value of this error alone is nearly 1/5 times as much as the resolution performance of a line width of 0.25 .mu.m. In view of a budget for superposition, this value is clearly beyond an allowable limit. It is, therefore, a serious problem for the art of optical exposure to suppress the intrashot error component.
In order to improve the precision of positional alignment of the patterns to be superposed on each other, therefore, the intrashot error component must be adequately controlled. Meanwhile, the projection optical system used for the stepper generally has a function of correcting only such components that are rotationally symmetric with respect to an optical axis.
However, in the actual processes in the manufacture of a semiconductor device, magnifying rates of expansion and contraction of the wafer caused by processes in the x and y directions, i.e., horizontal and vertical directions, sometimes vary and fluctuate among the results of these processes according to the directivity of patterns to be printed. The fluctuations of the rates of expansion and contraction impose some limitation on the efforts to improve the precision of the overall position alignment. As regards the arrangement for correcting the rates of vertical and horizontal expansion and contraction independently of each other, a stepper disclosed in Japanese Laid-Open Patent Application No. Hei 10-242048 is arranged to make correction by shifting an aspheric surface.
Another important subject for improvement in the precision of alignment lies in factors on the side of the reticle. One of the main factors on the side of the reticle is the perpendicularity of a pattern formed on the reticle. The perpendicularity error is not caused by an exposure apparatus or by a wafer process but is caused by fluctuations of the perpendicularity of a reticle pattern drawing apparatus.